1. Field
An embodiment of the present invention relates to a data driver, and more particularly, to a data driver that controls the slew rate of an output buffer provided in the data driver and an organic light emitting display having the same.
2. Description of Related Art
Recently, various flat panel displays that are light in weight and smaller in volume than that of an equivalent cathode ray tube, have been developed. Here, a flat panel display can be a liquid crystal display, a field emission display, a plasma display panel, a light emitting display, etc.
A flat panel display typically includes a display panel, a scan driver, and a data driver. The scan driver outputs scan driving signals sequentially to a plurality of scan lines formed on the display panel, and the data driver outputs R, G, and B image signals to the data lines of the display panel.
Here, the respective image signals output from the data driver are amplified at a set or predetermined level through output buffers to be output to the data lines of the display panel.
FIG. 1 is a perspective diagram showing one example of an operational amplifier that constitutes an output buffer provided in a data driver of a related art.
The output buffer 10 shown in FIG. 1 has a folded cascade operational amplifier circuit 11 having a rail to rail input terminal structure and an output circuit 12 having a common drain amplifier and a compensation capacitor C.
The folded cascade operational amplifier circuit 11 amplifies the difference of signals between a first input terminal Vin+ terminal and a second input terminal Vin− terminal, and the output circuit 12 amplifies and outputs the signals output from the folded cascade operational amplifier circuit 11.
The folded cascade operational amplifier circuit 11 has a PMOS current bias circuit 13 and an NMOS current bias circuit 14. Herein, the PMOS current bias circuit 14 has a PMOS transistor MP1, wherein the PMOS transistor MP1 is driven by bias voltage VBP generated from a bias voltage generator to supply bias current IBP1 to the folded cascade operational amplifier circuit 11.
Moreover, the NMOS current bias circuit 14 has an NMOS transistor MN1, wherein the NMOS transistor NM1 is driven by bias voltage VBN generated from the bias voltage generator to supply bias current IBN1 to the folded cascade operational amplifier circuit 11.
The slew rate of the output of the output buffer 10 may be represented by
                    I        BN            -              I                  BP          ⁢                                          ⁢          1                            2      ⁢      C        .
Here, the slew rate refers to maximum variation of the output voltage per unit time, that is, the slew rate refers to the instant slope (=value differentiated for time) of the output voltage when describing the output voltage using a graph for time.
In the data driver provided in a display, many characteristics are determined by the output buffer that outputs driving voltage to a display panel, wherein the slew rate of the output buffer among these characteristics have a great influence on the driving current of the data driver.
In particular, when demux Demux is used for the reduction in input time due to the larger panel size of the modern display and/or for the reduction in costs by reducing the size of the driver IC of the display, there is a demand for the reduction in the slew rate of the output buffer.
However, in the related art, the slew rate of the output signal output from the data driver depends on the bias current IBP1 and IBN1 of the output buffer and the compensation capacitor C, which limit how much the slew rate can be reduced.